The use of antiretroviral therapy (ART) typically results in reduction of plasma viral loads to below detectable levels in human immunodeficiency virus (HIV)-infected individuals. However, infection with HIV persists despite suppressive ART and treatment interruption results in rapid viral rebound. In absence of ART, CD8+ T cells have been shown to inhibit virus replication during HIV infection and depletion of CD8+ lymphocytes in vivo during simian immunodeficiency virus (SIV) infection of rhesus macaques (RMs) results in increased viral loads. The precise role of CD8+ lymphocytes in controlling virus replication and/or production during continuous, highly active ART is unknown. Understanding the mechanisms controlling HIV/SIV reservoir dynamics under ART, and particularly the role of host immune responses, is critical to design effective strategies to reduce the size of these reservoirs and promote HIV/SIV remission. In this proposal, we will build upon our preliminary data indicating that CD8+ lymphocytes act in concert with ART to maintain virus suppression. Using the highly relevant SIV/RM model, we will answer three important questions regarding the mechanism(s) of CD8+ lymphocyte-mediated virus suppression. First (Aim 1), we will determine if the antiviral effect of CD8+ lymphocytes is present in SIV-infected ART-treated RMs with prolonged suppression of viremia (as an extension of our preliminary results demonstrating this effect in the setting of short-term virus suppression). In this study, we will perform CD8 depletio in SIV-infected RMs treated with ART for at least one year to more closely mimic long-term ART-treated HIV-infected individuals with a stable virus reservoir. Second (Aim 2), we will determine if the observed antiviral effect of CD8+ lymphocytes under ART is mediated by CD8+ T cells vs. CD8+ NK cells. This critical experiment is made possible by a newly available monoclonal antibody (mAb) that targets cells expressing CD8? (i.e., CD8??+ T cells, but not CD8?+ NK cells) for depletion. Third (Aim 3), we will quantify the contribution of CD4+ T cell activation/proliferation to the increase in viremia that follows in vivo CD8 depletion. By using a neutralizing anti- IL-15 mAb together with CD8 depletion we can selectively block homeostatic CD4+ T cell activation and measure subsequent virologic outcomes. This work will allow us to understand how the host antiviral cellular immune response works in concert with ART to suppress SIV replication and/or production. These results will refine and clarify the evidence to explore cure-directed interventions, such as therapeutic vaccination and checkpoint blockade inhibitors, aimed at boosting the virus-specific CD8+ lymphocyte response in ART-treated HIV-infected individuals.